off the top of my head, women aren't really considered as heavily for roles that are more-so about 'leadership,' eg CEO or other executive positions. i doubt this is nearly as significant now as it has been in the past, but it's still a problem. but of course, it's a symptom and not a factor.
essentially it just kinda comes back to the whole traditional gender roles thing where women are expected to be homemakers. as a result, women are more so seen as useful for expressive tasks like teaching, while men are more so seen as useful for instrumental tasks like principal. obviously, of course, support for the idea of traditional gender roles has been on the decline for at least half a century now, and so the division caused has diminished as well, but it's still an issue for people on both sides. just as women might not be as likely to become a CEO, a man probably wouldn't be as likely to get a job dealing with children. it goes both ways.